/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution
{
public:
    ListNode* swapPairs(ListNode* head)
    {
        if (head == nullptr || head->next == nullptr) return head;

        ListNode* twonode = head->next->next;
        ListNode* newhead = swapPairs(twonode);
        ListNode* next = head->next;
        next->next = head;
        head->next = newhead;

        return next;
    }
};